Periodic monitoring of blood glucose levels is needed for the diagnosis and prophylaxis of diabetes mellitus. It is easily performed by individuals using strip-type biosensors designed for hand-held reading devices. Many commercial biosensors exploit the following reaction to quantify the glucose levels in whole blood samples:glucose+GOx−FAD→gluconic acid+GOx−FADH2GOx−FADH2+Mox→GOx−FAD+Mred(wherein, GOx represents glucose oxidase; GOx−FAD and GOx−FADH2 represent an oxidized state and a reduced state of glucose oxidase, respectively; and, Mox and Mred denote the oxidized and reduced electron transfer mediator, respectively. As shown in the reaction, glucose is oxidized to gluconic acid by reducing GOx−FAD to GOx−FADH2. The reduced glucose oxidase transfers electron(s) to the mediator Mox and to the working electrode. The series of reaction cycle is driven by the anodic potential applied at the working electrode, and the redox current proportional to the level of glucose is measured.
Although electrochemical biosensors are conveniently used for monitoring and controlling the blood glucose levels, their accuracies are greatly affected by the presence of various easily oxidizable species (e.g., ascorbic acid, uric acid, acetaminophen, and etc.) in blood samples. Another serious measurement bias results from blood hematocrits. The interference from oxidizable species may be reduced by employing an electron transfer mediator that has lower oxidization potential than the interfering substances in the reagent layer. However, few practical solutions have been proposed to reduce the measurement bias from blood hematocrits. They teach the use of an additional hematocrit separation or erythrocyte exclusion layer (JP 1134461, JP 2000338076, and U.S. Pat. No. 5,658,444) dispensed on the reagent layer, printable reagent/blood separation paste formulated with a silica filler (U.S. Pat. No. 6,241,862 B1), and a chemometric correction method combined with the double excitation potentials (WO 01/57510 A2). The disclosed methods, however, cannot be realized by simple dispensing of the reagent cocktail on the working electrode, requiring either extra steps to the manufacturing process or a large loss of the reagents in printing the reagent layer.
A large hematocrit level-dependent bias may lead to an erroneous judgment for those who regularly monitor their blood glucose levels with disposable biosensor strips, causing even the loss of their lives.